In a conventional magneto-optical recording method a vertically magnetized film composed of metal magnetic material coated onto a substrate made of glass, plastic, ceramic or other material serves as a recording medium, and recording and reproducing operations on and from the recording medium are carried out as described hereinbelow.
In the recording operation, first of all initialization is performed by arranging a magnetization direction of the recording medium to a predetermined direction (upward direction or downward direction) according to an external magnetic field or the like. Then a temperature of a recording portion where the recording is to be carried out is raised to a point above the vicinity of the Curie temperature or to a point above the vicinity of the magnetic compensation temperature by projecting a laser beam on the recording area. As a result, a magnetic coercive force at the recording portion becomes zero or substantially zero. The magnetization direction is then reversed by applying the external magnetic field, based on the fact that this external magnetic field has a reverse magnetization direction with respect to the magnetization direction of the initialized recording medium. After that, when the projection of the laser beam is stopped, the recording portion of the recording medium returns to room temperature. A reversed magnetization is thus fixed and information is recorded thermomagnetically.
In the reproducing operation a linearly polarized laser beam is projected onto the recording medium. A polarization plane of reflected light or transmitted light from or through the recording medium has a direction of rotation that varies according to a magnetization direction of the recording portion (Kerr effect or Faraday effect). Information is optically read out according to differences in the direction of rotation of the polarization plane.
Recording media used in the magneto-optical recording method have been noted as large capacity memory elements of a re-writable type. However, there are two methods for re-writing over the information recorded on the recording medium, described in (i) and (ii) hereinbelow.
(i) A method wherein a deletion of the previously recorded information is performed by initializing the recording medium once again.
(ii) A method wherein a recording medium or an external magnetic field generating device is improved so that overwriting, i.e., direct re-writing of the information without performing the deletion, may be carried out.
If the method (i) is adopted, either an initialization device or two heads must be installed, thereby causing a rise in cost. Moreover, when only one head is provided and the deletion is performed according to the method (i) the deletion operation is inefficient because the deletion operation requires the same amount of time as the recording operation.
On the other hand, if the method (ii) is adopted and an improvement in the recording medium is carried out, it is generally accompanied by difficulties in controlling film composition and film thickness and so on. For this reason, the most suitable method in (ii) is regarded as improving the external magnetic field generating device, i.e., switching a direction of the external magnetic field at high speed while keeping an intensity of the laser beam constant.
In order to switch a direction of the external magnetic field at high speed it is necessary to have a low impedance by making a coil and a coil core of the external magnetic field generating device extremely small. However, a generating area of the magnetic field becomes smaller as a coil and a coil core are made smaller. In order to counteract this, a magnetic head and a recording medium must be brought closer to each other. Thus, as shown in FIG. 7, generally a start-and-stop-in-contact head of a sliding type is adopted. The start-and-stop-in-contact head permits the external magnetic field generating device to glide over a recording medium (not shown). The magnetic head (not shown) is provided on a slider 1 The slider 1 is supported by a suspension 3 made of a leaf spring or the like, a base section of the suspension 3 being joined to a supporting base 2. The slider 1 is suspended from the suspension 3 and thus a depressing force is exerted on the slider 1 in a vertical direction with respect to a surface of the recording medium. When the recording medium is rotated the slider 1 is designed to float above the surface of the recording medium.
A constant floating gap between the slider 1 and the recording medium is maintained due to the fact that a floating force balances with the depressing force. The floating force is exerted upwards on the slider 1 by an air flow between the slider 1 and the recording medium. The depressing force is exerted downwards on the slider 1 by the suspension 3. The start-and-stop-in-contact head of this type is used for conventional hard disks for computers as well. In the case of the hard disks the floating gap is of a submicron order. However, when the recording medium is a magneto-optical disk, a floating gap of 5.mu.m-15.mu.m is necessary since magneto-optical disks are transportable. This increased gap is necessary for the following reasons.
Since magneto-optical disks are transportable, the likelihood of dirt and so on sticking on the surface increases. If the slider 1 and the magneto-optical disk approach too close to one another they may collide. For example, if a magneto-optical disk has an uneven surface the air flowing past the slider is disturbed. As a result, the floating force exerted by the air on the slider 1 changes and the gap between the magneto-optical disk and the slider 1 no longer remains constant.
Thus, in the case of a transportable magneto-optical disk the gap of 5.mu.m-15.mu.m, a larger gap than the gap required for a hard disk, is necessary. Therefore, as shown in Table 1, the floating gap changes depending on a relative velocity between the magneto-optical disk and the slider 1. For example, if the relative velocity doubles, the floating gap increases by a substantial one and half times (here, the dimensions of the slider 1 are 6mm.times.4 mm). Thus, the floating gap increases as an outer part of the magneto-optical disk is approached from an inner part since the relative velocity increases as the outer part is approached. Consequently, a magnetic field intensity applied to the magneto-optical disk changes according to a radial position on the magneto-optical disk, and the recording operation cannot be carried out under constant conditions.
TABLE 1 ______________________________________ (a relationship between the relative velocity and the floating gap) V F 10 m/s 20 m/s ______________________________________ 5 gf 6.5 .mu.m 10 .mu.m 10 gf 4 .mu.m 6.5 .mu.m ______________________________________ Where, F: depressing force due to the suspension V: headmedium relative velocity